Continuously variable transmissions (CVTs) are commonly used on a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts, scooters, all-terrain vehicles (ATV), etc. They often comprise a drive pulley mechanically connected to a motor, a driven pulley mechanically connected to wheels, tracks or caterpillars, possibly through another mechanical device such as a gearbox, a drive train and a trapezoidal drivebelt transmitting torque between the drive pulley and the driven pulley. A CVT changes the ratio within certain limits as required by the operating conditions to yield a desired motor rotational speed for a given driven pulley rotational speed, the latter being generally proportional to the vehicle speed. A CVT may be used with all kinds of motors, for instance internal combustion engines, electric motors, windmills, etc. CVTs can also be used with other machines that are not vehicles.
Each pulley of a CVT comprises two members having opposite conical surfaces, which members are called sheaves. One sheave, sometimes called “fixed sheave”, can be rigidly connected to one end of a supporting shaft while the other sheave, sometimes called “movable sheave”, can be free to slide and/or rotate with reference to the fixed sheave by means of bushings or the like. The conical surfaces of the sheaves apply an axial force on the drivebelt. Moving the sheaves axially relative to each other changes the drivebelt operating diameter, thus the ratio of the CVT.
In order to transmit the motor torque, an axial force has to be applied in the driving and the driven pulleys. These axial forces can be generated by a plurality of possible mechanisms or arrangements. In a legacy mechanical CVT, the axial force in the drive pulley is often generated using centrifugal weights, spring and ramps. In a legacy driven pulley, this force is often generated using cam surfaces and a spring.
Generally, at a low vehicle speed, the operating diameter of the drivebelt at the drive pulley is minimal and the operating diameter at the driven pulley is maximal. This is referred to as the minimum ratio or the minimum ratio condition since there is the minimum number of rotations or fraction of rotation of the driven pulley for each full rotation of the drive pulley.
As the vehicle speed increases, so does the driven pulley rotational speed. For a given operating condition, a certain motor rotational speed is desired, thus a desired ratio can be calculated. The CVT actuation mechanism is provided to set the CVT to the appropriate ratio.
Generally, when the rotational speed of the drive pulley increases, its movable sheave moves closer to the fixed sheave thereof under the effect of an actuation mechanism, for instance a centrifugal mechanism or another kind of actuation mechanism. This constrains the drivebelt to wind on a larger diameter at the drive pulley. The drivebelt then exerts a radial force on the sheaves of the driven pulley in addition to the tangential driving force by which the torque received from the motor is transmitted. This radial force urges the movable sheave of the driven pulley away from the fixed sheave thereof, thereby constraining the drivebelt to wind on a smaller diameter at the driven pulley. A return force, for instance a return force generated by a spring of the driven pulley and/or by another biasing mechanism, often counterbalances the radial force. It may also be counterbalanced by a force generated by the axial reaction of the torque applied by the drivebelt on the driven pulley, which force often results from the presence of a cam system and/or another biasing mechanism that tend(s) to move the movable sheave towards the fixed sheave as the torque increases. A cam system may comprise a plurality of ramp surfaces on which respective followers can be engaged. The followers can be sliding buttons or rollers, for instance. The set of ramp surfaces or the set of followers is attached to the movable sheave. The other set is directly or indirectly attached to the fixed sheave and is in a torque-transmitting engagement with the main shaft supporting the driven pulley. The closing effect of the cam system on the drivebelt tension is then somewhat proportional to the torque received from the motor.
Generally, at the maximum vehicle speed, the ratio is maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each full rotation of the drive pulley.
When the vehicle speed decreases, the rotational speed of the drive pulley eventually decreases as well since the rotational speed of the motor will decrease at one point. Ultimately, there is a decrease of the winding diameter at the drive pulley and a decrease of the radial force exerted by the drivebelt on the sheaves of the driven pulley. The driven pulley is then allowed to have a larger winding diameter as the spring and/or another biasing mechanism move(s) its movable sheave closer the fixed sheave.
Some CVTs are provided with an integrated clutch function. The clutch function can be on the drivebelt or be provided by a mechanism incorporated in the CVT. For instance, when the CVT has a clutch function on the drivebelt, the opposite walls of the fixed sheave and the movable sheave of the rotating drive pulley can be designed to be sufficiently apart that they are not in a driving engagement with the sides of the drivebelt. The drivebelt is then not moving and some models of drive pulleys have a bearing provided between the two sheaves. The outer race of such bearing supports the drivebelt when the drive pulley is in a disengaged position. Then, when the operating conditions are such that clutching is required, the actuation mechanism of the drive pulley moves the sheave walls closer relative to each other. The sheave walls eventually make contact with the sides of the drivebelt. At this point, an axial force is applied by the actuation mechanism on the drivebelt. The amount of torque transferred to the drivebelt is somewhat related to this axial force applied by the actuation mechanism. At one point, enough friction/force is generated between the sheave walls and the drivebelt to produce a significant force transfer between the driveshaft and the drivebelt, thereby causing torque from the motor to be transferred as a driving force on the drivebelt. This driving force is transferred to the driven pulley of the CVT.
Generally, torque applied on the drivebelt will result in vehicle acceleration at some point. The drivebelt will then accelerate in relation to vehicle speed. At start-up, the slippage between the drive pulley sheaves and the drivebelt is high, but decreases as the drivebelt accelerates, to the point where it becomes negligible and the drive pulley is considered fully engaged.
Electronically controlled CVTs are advantageous because they do not relate on the centrifugal force generated by the rotation of the sheaves like legacy CVT mechanical actuation mechanisms. In contrast, an electrically actuated CVT uses an electric motor and an adapted gearbox to set the CVT ratio. This provides the flexibility of using a specific CVT ratio in reaction of predetermined conditions regardless of the centrifugal force applied on the pulleys. Despite the advantages provided by an electronically controlled CVT, it is appreciated that the assembly of an electronically controlled CVT represents some challenges or benefits not encountered with legacy CVTs.
An electronically controlled CVT uses an assisting mechanism to manage the CVT ratio by changing the width of the drive pulley without solely relating on centrifugal forces. The assisting mechanism can be secured to the drive pulley preferably on the side opposed to the engine. The assisting mechanism can be operatively secured to the engine's drive axle without rotating therewith. At least a portion of the assisting mechanism moves along the engine's drive axle with the change in width between the drive pulley sheaves. This combined movement requires an adequate mechanical structure adapted to sustain fast repetitive movements under significant vibrations and mechanical loads.
Gears and axles are arranged in a complex operating layout in the electronically controlled CVT where small volume and low weight are key. Other considerations also need to be kept into account. For instance, the CVT should be easy to assemble, inexpensive to produce and minimize chances of errors during the assembly process. Additionally, the design of the electronically controlled CVT components should consider a variety of criterion like the mechanical resistance, the weight, the moment of inertia, the method of assembly and the manufacturing material in addition to the effect on the cost of the assembled final component.
The entire drive system, from the engine to the wheels in the case of a wheeled vehicle, needs to be sized and designed to sustain normal operating loads applied thereto. It is likely that such a drive system would experience a significant failure rate during typical use. In contrast, designing the entire drive system in consideration of the maximum operating load ensures the drive system be reliable under all possible loads despite maximum loads will be seldomly experienced under typical use. Such a more robust drive system uses bigger and heavier components to sustain possible high peak loads. This additional material in the drive system increases the size and the weight on the vehicle. The additional weight carried by the vehicle has the effect that more energy is required to accelerate and decelerate the vehicle. Some components of the drive system are rotating and are therefore requiring even more energy to accelerate, be maintained in rotation and to decelerate their rotating movement given their higher moment of inertia. Heavier rotating parts (or parts having a higher moment of inertia) are less energy efficient and increase the vehicle's energy consumption along the entire useful life of the vehicle.
Another drawback of legacy CVTs is that they relate on a centrifugal clutch to disengage from the drive mechanism. In other words, an electronically controlled CVT generally needs a separate clutch to completely disengage from its rotating power source. One particular problem with this type of drive system has been that, when the drive system is subjected to significant impact loads, such as those that occur, for example, when the vehicle jumps and the airborne wheel(s) accelerates on driver's demand before touching back the ground. These impact loads stem from the difference between the speed of the vehicle that is jumping and the circumferential tangential velocity of the airborne wheels of the vehicle. Much important torque peaks are sustained by the drive system when the vehicle lands after the jump and abruptly touches the ground to (almost) instantaneously bring back the circumferential tangential velocity of the wheels equal to the speed of the vehicle. The wheel acceleration just before a jump is sometimes at wide-open throttle and acceleration of the wheels is thus very fast when leaving the ground. These peak mechanical loads in the drive assembly are caused, in particular, because of the conjunction of high moment of inertia of the CVT and the drive assembly and high deceleration rate of the drive train.
Conventional drive assemblies, such as the one disclosed in U.S. Pat. No. 3,997,043, include an overload clutch disposed between the transmission and the wheels of the vehicle. The overload clutch disengages when a mechanical torque transmitted therethrough exceeds a predetermined value to try preventing damaging the drive train. On the other hand, the addition of a clutch undesirably increases the moment of inertia of the drive system.
Therefore, a need has been felt for an improved electronically controlled CVT over the prior art. It is therefore desirable to provide an electronically controlled CVT having a torque-limiting mechanism and method thereof adapted to prevent having to significantly oversize the drive train of a vehicle. Another need, inter alia, has been felt over the existing art for an electronically controlled CVT adapted to limit the torque transmitted to the drive train of a vehicle without adding more rotating mass to the drive train by matching the circumferential tangential velocity of an airborne wheel with the absolute vehicle speed.